October 2006 Design Tip

Resin, Aim, Fire

Imagine that you’ve embarked on a home DIY project of some sort. You’ve drawn meticulous plans and taken them to the materials desk at your local DIY retailer. The consultant asks, “What sort of material will you be using: wood or metal,” and you reply, “Oh, I hadn’t thought about it.” Sounds pretty unlikely, since the two materials have completely different characteristics and a plan for one wouldn’t work very well for the other. But that could be exactly what you are doing if you design a plastic part without having at least some idea of the sort of resin you plan to use.

While all injection moulding resins share certain characteristics, they vary a great deal, not just in their performance characteristics but in their mouldability traits as well. Of course there are principles of good design that apply to any mould, and there are others that apply more to particular types of resin. These differences can have a significant impact on your design.

One obvious difference among resins is strength. As a result, a design that might be perfect for one resin could be grossly over- or underbuilt for a stronger or more fragile resin. Another set of resin-specific behaviors are those of glass-filled resins. As these composite materials flow into a mould, the glass fibers orient themselves along the direction of resin flow. This can affect the strength of the finished part as well as relative shrinkage parallel to, and perpendicular to, resin flow.

In some cases, an application requires a resin with specific traits like strength, transparency, flexibility, corrosion resistance, or electrical resistivity. That can limit your choice of resins…

Even without fill, the shrink rate of a resin during cooling can be an important consideration. While it is always preferable to design a part without thick areas that would be susceptible to shrinkage, you can also control shrinkage by choosing the right resin. In short, the best parts result from an optimal combination of design and material choices.

So, you may ask, why does it matter whether the design is developed to fit the material or the material is chosen to fit the design. The answer is that sometimes it doesn’t matter. But sometimes it does.

In some cases, an application requires a resin with specific traits like strength, transparency, flexibility, corrosion resistance, or electrical resistivity. That can limit your choice of resins, forcing you to design your part around the requirements of those resins that meet your must-have requirements. In other cases, there may be resins that will work with a less-than-optimal design but are expensive. You then face the choice of having to pay more for material for the entire production run or having to revise your design to allow a wider choice of resins. And while quick-turnaround prototyping from Protomold can speed up redesign and help hold down costs, design changes still cost valuable time in getting your product to market.

Bottom line: you’re going to have to choose a resin at some point in the process. Doing so early on gives you valuable information that can help you design lighter, stronger, more mouldable, and generally better parts and reach your final goal faster and more economically. You can always fine tune your choice as the process moves forward.

For more information visit our Design Guidelines at: /designguidelines/choosingaresin/